1. Field of the Invention
The invention relates to circuitry for controlling the output illumination level of gas discharge lamps and more particularly to circuitry having load side control and improved lamp current waveforms utilizing a circulating inductor circuit in parallel with a controlled impedance coupled between the ballast and the gas discharge lamps.
Numerous techniques have been proposed for controlling the output illumination level of gas discharge lamps. Present day objectives are directed to efficient energy use, and exemplifying such applications are control circuits for lamp dimming in response to selected illumination levels. One such system is illustrated in U.S. Pat. No. 4,197,485. Principal deficiencies impeding the development of this technology have been (1) dimming systems have, heretofore, generally reduced the net efficiency (lumen output/wattage input) of the lighting system; (2) the dimming circuitry, when sufficiently sophisticated to provide efficient dimming, becomes costly and burdensome. In contrast, the present invention is directed to a simple, yet efficient, method for illumination control of gas discharge lamps.
An alternative commonly employed to increase overall efficiency in dimming systems is to convert line frequency to higher frequencies. Illustrative of this technique are U.S. Pat. Nos. 4,207,497 and 4,207,498. In contrast, the present invention operates at line frequency. To enhance efficiency, the invention employs a novel configuration of load side control complemented by an inductive circulating current load to achieve circuit simplicity while maintaining an excellent power factor, illumination control of 10 to 1 dimming, excellent current crest factor and reduced lamp current and ballast loss. An attendant advantage of the circuit simplicity is the ready adaptation of the circuit to the physical housing of the conventional gas discharge lamp, an important economic and aesthetic concern.
2. Summary of the Invention
The invention is directed to an apparatus and method of controlling the output illumination level of gas discharge lamps such as fluorescent lighting systems or the like. Load side control is provided by timed interval controlled impedance, serially coupled between the ballast and the lamp(s). An inductor is coupled in parallel relation to the controlled impedance. The inductor provides a current path between the power source and the lamp(s) at least during that portion of the AC waveform where the controlled impedance is in a substantially non-conductive state. The novel configuration facilitates the use of conventional magnetic ballast illumination control in a plurality of ballast/lamp arrangements, in the illumination range of 10% to 100% of full intensity illumination with substantially no reduction in the cathode heating voltage supplied to the lamp(s). An attendant advantage of the circulating inductor configuration is a reduced blocking voltage requirement for the controlled impedance, further simplifying component requirements.